Basic Highway Design

Many of the geometric design standards and procedures are derived from A Policy on Geometric Design of Highways and Streets , published by the American Association of State Highway and Transportation Officials (AASHTO) in 1994. Sight distance should provide the driver with adequate warning about possible hazards or obstructions in the roadway, and of changes in the alignment of the road that lies ahead. On the tangent sections of roads, which are straight as an arrow, traffic speed is primarily limited only by road surface conditions, lane width, vertical grade, passing opportunities, the capabilities of the vehicle, and the desires of the driver. AASHTO discusses design service flow rates and provides a table that suggests a starting point for selection of appropriate Level of services for various types of highways. The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of the highway.